1. Field of the Invention
This invention relates to a novel reaction system for producing a polyurethane including a unique catalyst system effective therefor. Such a reaction system finds use for example to form a sealant coating or strip.
2. Discussion of the Related Art
It is known to formulate polyurea-urethane compositions (e.g., sealants, coatings, foams, and the like) as two pack systems. One pack includes the isocyanate-reactive components such as polyols, together, typically with a catalyst and other customary additives, while the second pack includes the polyisocyanate. The catalyst is separately packaged from the polyisocyanate in order to prevent premature gelation of the latter component. The two packs are normally mixed immediately prior to application of the coating. Upon mixing the two packs, the hydroxyl groups of the polyol chemically react with the isocyanate groups of the polyisocyanate, ultimately leading to gelation. At gelation, the reaction mixture rapidly loses its fluidity with an attendant pronounced increase in viscosity. The time associated with reaching the gelation point also is often referred to as the "potlife".
While bismuth carboxylate catalysts are known, their catalytic effect in promoting the cure of isocyanate-containing compositions is such that the rate of cure is generally linear. While such rates of cure are desirable for some applications, a curing rate is also desirable for many applications characterized by providing a slow incipient build-up of viscosity but with the reaction mixture nonetheless completely curing or curing to a tack-free state within a relatively short period of time.
An example of such an application is for the cure of a two-pack or part urethane adhesive composition, wherein a "pot-life" is desired during which the composition may be handled and/or applied to a substrate before it substantially cures. In this regard, certain mercury compounds are known to be useful as latent catalysts, but their use is not desirable for environmental reasons.
One solution is taught by U.S. Pat. No. 4,788,083 which describes an activatable/complexed catalyst system which provides an extended potlife yet is readily activatable with vaporous amine or heat. The catalyst is a complexed catalyst generally comprising the reaction product of a tin and/or bismuth polyurethane catalyst with a molar excess of a complexing agent selected from a mercapto or polyphenol compound. However, the use of vaporous amine and/or heat activation methods to incite rapid cure may present problems on a commercial scale. For example, accurate dosage of the amine activator can be troublesome, which can adversely impact the appearance and resistance to yellowing. Use of heat activation requires an oven, which is impractical for operations involving large objects, such as installing and sealing a glass windshield into a car.
Laid-Open Japanese patent application JP H1-213,382 (Sakada et al.), Oct. 15, 1987, describes two component polyurethane adhesives wherein organic tin compounds are used in conjunction with multi-functional mercaptan compounds to delay the onset of cure and to provide "adaptability" (a period of workability) without undue lengthening of the adhesive hardening time.
European Patent Application Publication No. 0 454 219 (Oct. 30, 1991), describes a polyurethane system which is stated to eliminate the need for the amine or heat activation required in U.S. Pat. No. 4,788,083, by use of a polyol component having an acid value of about 5 or less. The publication describes polyurethane systems having extended potlifes of multiple hours.
Also, commonly assigned U.S. Pat. No. 5,250,651 discloses a sealant composition useful for insulation displacement connector enclosures. The sealant composition in U.S. Pat. No. 5,250,651 is synthesized by a reaction between polyether species and isocyanate species, requiring a stoichiometric deficit of the isocyanate reactant, that results in a gel-like polymer which is lightly cross-linked through urethane linkages.
Metal ion catalysis and inhibition of isocyanate reaction in the presence of mercaptans has been studied by J. Robins in Metal Ion Catalysis In Mercaptan Isocyanate Reactions, Advances in Urethane Science and Technology, Volume 12, Technonmic Publishing Company, Lancaster, Pa. 17604, (1993), pp. 25-58. J. Robins classifies metal ions in three categories on the basis of their ability to complex with mercapto cure inhibitors and yet act as catalysts after controlled induction period where no catalytic action is observed. Robins' three classes are: (1) metal ions which are good catalysts (e.g., In.sup.+3, Tl.sup.+4, Sn.sup.+2); (2) metal ions which exhibit fair catalytic activity (e.g., Zn.sup.+2, Pb.sup.+2, Bi.sup.+3, Et.sub.2 Bi.sup.+); (3) metal ions which show a very low catalytic activity (e.g., Ca.sup.+2, Co.sup.+2, Cu.sup.+2); and (4) no catalytic activity (e.g., Li.sup.+, Na.sup.+, Al.sup.+3).
In a paper presented at UTECH '92 co-authored by John D. Arenivar (Shepard Chemical Co.) and Kurt C. Frisch et al. (Polymer Technologies, Inc., University of Detroit Mercy), entitled "Metal Carboxylates For Urethanes", a study is discussed of the kinetics of bismuth and/or zinc carboxylate-catalyzed urethane reactions by using model compounds for the reactants in separate non-competitive reactions. The data of this study is then described as having been used to successfully formulate flexible foams, catalyzed by bismuth and zinc carboxylate compounds in the absence of tertiary amine catalysts.
There also are many applications where a polyurethane sealant would be desirable which possesses a cure profile providing a sufficient induction period to tool and manipulate the contour the admixed sealant, if necessary, before the onset of cure. There also exists a need for a sealant that has a induction period that does not require special activation procedures, yet, in a relatively short period of time the coated workpiece can be quickly further handled or returned to service. An example of such an application is the installing of a glass windshield into a car frame.
That is, in many applications, such as installing a glass windshield into a car frame, rapid cures are desired without the need for special activators for the system. For instance, in a glass repair shop or vehicle assembly line setting for installing glass windshields, a glass sealant is desired which controllably cures or gels at a time that falls inside a time span or window beginning several minutes after admixture of the polyurethane-forming components to within about an hour after admixture. Such as cure profile would provide reasonably ample time to apply and perform any necessary tooling and shaping of the sealing coating on the surface to be sealed before the reaction mixture sets-up to an extent precluding such manipulation, while avoiding unduly long open times to allow the vehicle to be returned to service or further handled as soon as possible. The period of time spanning from the mixture of the polyurethane-forming components to the gel time is often referred to as the open time or induction time.